Xerographic transfer method



Dec. 29, 1959 L. E. VWALKUP XEROC-RAPHIC TRANSFER METHOD Filed Dec. 27, 1954 INVENTOR L euz's Z: Walk El"! aQ/W ATTORN EY United States Patent 01 XEROGRAPHIC TRANSFER METHOD Lewis E. Walkup, Columbus, Ohio, assignor, by mesne assignments, to I-Ialoid Xerox Inc., Rochester, N.Y.,

a corporation of New York Application December 27, 1954, Serial No. 477,810

3 Claims. (Cl. 9'6-1) This invention relates to xerographic processes and in particular to the transfer of electroscopic powder images after development, especially under conditions of high relative humidity.

In the art of xerography as disclosed in Carlson Pat ent'No. 2,297,691, it is usual to place an electrostatic charge on a xerographic member which is generally composed of a photoconductive layer overlying a conductive backing. This charged member is then exposed to copy to be reproduced and the electrostatic charge on the photoconductive insulated layer is selectively discharged, thereby leaving an electrostatic latent image on the photoconductive surface. This latent image may then be developed by depositing thereon electroscopic particles caring layer, or and preferably, it may be transferred to other material.

One of the usual methods of transfer is to cause particles of electroscopic material forming the developed image pattern to move from the insulating surface to another surface, for example, paper by the use of electrostatic forces. This transfertechnique is not particularly difiicult in ordinary circumstances. In the past, however, a number of difiiculties have been encountered in employing the electrostatic transfer process of xerography at high relative humidities. Such diificulties are to be expected, because xerography depends upon minutes charges of static electricity which can be drained away over the smallest leakage paths. The practical importance of the effect on transfer of humidity has become greater in recent years because of the increasing quality expected of xerographic products and because users are becoming more critical as the novelty of using the process wears off. It is common now to compare xerography with other commercial processes in terms of the results produced and it is expected to work almost perfectly under all times and under all conditions.

It has been found that for practical purposes using commonly known transfer procedure it is difficult if not impossible to transfer satisfactorily to paper or to offset duplicating mats when either is in equilibrium with air at high relative humidities. Objects and features of this invention are .to provide improved techniques of transfer of developed xerographic images to paper or to offset duplicating mats that yield effective and satisfactory results irrespective of the relative humidity conditions in existence at the time transfer is effected.

In its most common form electrostatic transfer comprises laying a sheet of paper over the developed toner image on the xerographic plate, delivering an electrostatic charge tothe paper, usually by means of a corona discharge, and, peeling the paper from the plate. This operation transfers to the paper approximately one-half of the image toner powder persent on the plate. This "ice procedure is satisfactory for most applications when humidity is .low. However, this transfer fails when the paper is made slightly electrically conductive as by the absorption of too much moisture, for example, under conditions of high relative humidity. .The reason for this failure with moistpaper is that the electrical charge on the paper drains away so rapidly that the potential cannot build up to the usual 300 to 600 volts necessary for adequate transfer of the powder image to the paper. While one way to improve transfer in such procedure would be to reduce the lateral electrical conductivity of the paper, this cannot be done as a practical matter except by keeping it dry until actually used. However, this is difficult from a packaging and storage standpoint and involves other objectionable features as well as added expense. This makes it preferable to provide other simple and inexpensive techniques for effecting the transfer irrespective of humidity conditions or of moisture content of the transfer paper or material.

While lateral conductivity in the moist paper is the principal cause of transfer failure at high relative humidities, this lateral conductivity would have no deleterious effect on electrostatic transfer if there were no paths over which the electrical charge could leak from the paper to the ground. The elimination or else the material reduction of such leakage has been proposed as a mode or technique for overcoming the difficulties of transfer to damp or moist paper under conditions of high relative humidity.

The leakage of the electrical charge from the moist transfer paper to ground affects the quality of the transfer because it reduces the potential on the paper. However, leakage in itself is not otherwise objectionable unless the current is great enough to damage the electrostatic plate. If, therefore, the potential on the paper during transfer can be maintained long enough to transfer the toner image, there would be no harm in some current leaking to ground during or after transfer. This invention contemplates provision of an arrangement for combating the effect of leakage and of the maintenance of sufiicient potential for effective transfer despite such leakage.

Other objects and features of this invention are the provision of an effective method of combating or overcoming the effects of leakage and of causing adequate transfer to moist transfer paper or other material during conditions of high relative humidity.

Further objects and features of the invention are the provisions of methods for effecting transfer under such conditions that are simple and inexpensive.

Further objects and features of the invention will become apparent from the following specification and from the accompanying drawing which is a diagrammatic illustration on an exaggerated scale of an arrangement for practicing this invention.

Referring to the drawing, the reference character 10 denotes generally a xerographic plate or element comprising as usual a backing 11 of I conductive material such as aluminum and provided on one surface with a photoconductive coating 12, for example, of amorphous selenium. As is known, a plate 10 of this character, as

image 13 now is ready for transfer preferably to transfer material 14 such as a paper sheet or a mat of offset material. With ordinary or conventional transfer procedure, ambient atmospheric conditions at the time of transfer to the material 14 are found to materially offset the quality of transfer to the material 14 particularly when high relative humidity prevails.

In order to counteract the effect of high relative humidity, and to secure fine quality transfers irrespective of its prevalence at transfer time, the practice in accord with the present invention is to apply a thin conductive foil 15 of aluminum or the like to the surface of the transfer sheet 14 and to apply a pressure pad 16, for example, of rubber, to cover the foil 15 so as to form a sandwich 17 consisting of the transfer member 14, the foil 15 and the pad 16. The sandwich 17 is laid over and in contact with the image 13 on coating 12 of the element 10. The foil 15 is then connected to the positive pole of a high voltage D.C. source 18 through a switch 19. The negative pole of source 18 is connected to the aluminum backing member 11 and to ground.

The foil 15 while thin is not of itself flexible enough to conform entirely to the contours of the transfer member or sheet 14 overlying image 13. However, the weight of the pad 16 is selected to provide sufficient pressure to the foil 15 to cause its proximate contour conformity to the contour of the transfer member or sheet 14 lying on the image 13. This pressure may be augmented if desired by any conventional arrangement if needed. Thus, the conductivity of foil 15 distributes or supplies current from the source 18 uniformly and rapidly to all points of the transfer member or mat 14 when the switch 19 is closed, charging the mat all over sufficiently and rapidly enough to cause electrostatic transfer to its surface of the powder of image 13 despite the fact that electrical resistance between the member 14 and ground due to the moisture content of member 14 may be so low that lateral conduction through member 14 to ground can occur. The provision of the foil 15 pressed into substantial contour conformity with transfer member 14 by the pad 16 provides for distribution of copious amounts of electrical charge to all portions of the member 14 simultaneously from the source 18 which provides a potential of approximately 1000 volts. This uniformly distributed charge applied simultaneously to all areas of the mat 14 is found entirely sufficient to counteract charge leakage and dissipation therein caused by lateral conductivity through the moist member 14. In other words, the 300 to 600 volts potential necessary for adequate transfer has time to build up in all areas of the transfer member because of the rapid simultaneous application of 1000 volts to all its areas through foil 15 before lateral conductivity below the transfer potential due to moisture content of the transfer member 14 can act to dissipate and reduce the distributed high potential below the value necessary for effective transfer.

Promptly after transfer has been effected switch 19 is opened and the sandwich 17 is separated from the element with the powder image adhered electrostatically to the transfer mat or member 14. It may thereafter be fixed permanently to the said member as by heat or vapor fusion or by other known procedures.

It can be seen, therefore, that the method of this invention involves generally effecting contact between a powder image on a xerographic element and a transfer member and the simultaneous application of a massive potential to the entire surface of a transfer member through a conductive film in surface contact therewith, so that any lateral conductivity of the transfer member due to its moisture content under conditions of high relative humidity does not dissipate the applied massive potential in specific areas quickly enough to a value below that needed for effective electrostatic transfer so that electrostatic image transfer to the transfer member takes place.

With the procedure described, effective transfer has been practiced with the transfer member 14 and transfer apparatus in equilibrium with air at F. and 85 percent relative humidity.

While specific arrangements have been described, variations in detail Within the scope of the claims is possible and is contemplated within the scope of the appended claims. There is no intention, therefore, of limitation to the exact details described and shown.

What is claimed is:

1. In the process of xerography wherein an electrostatically charged, flat xerographic plate comprising a photoconductive insulating layer on a conductive backing is exposed to a light image to form an electrostatic latent image on the plate surface corresponding to the light image and the plate is developed with oppositely charged particles of developing material to form a xerographic powder image on the plate surface corresponding to the light image and the powder image is transferred to a transfer sheet superposed thereon, the improvement for effecting said powder image transfer under conditions of high relative humidity from the xerographic plate to a flexible transfer sheet having the property of high lateral conductivity under conditions of high relative humidity comprising the steps of superposing a conductive sheet on the flexible transfer sheet overlying the powder image, pressing the conductive sheet into intimate surface contact with the flexible transfer sheet, and establishing a potential difference between the transfer sheet and the plate by forming an electrical circuit between the conductive sheet and the conductive backing of the xerographic plate.

2. In the process of xerography wherein an electrostatically charged, flat xerographic plate comprising a photoconductive insulating layer on a conductive backing is exposed to a light image to form an electrostatic latent image on the plate surface corresponding to the light image and the plate is developed with oppositely charged particles of developing material to form a xerographic powder image on the plate surface corresponding to the light image and the powder image is transferred to a transfer sheet superposed thereon, the improvement for effecting said powder image transfer under conditions of high relative humidity from the xerographic plate to a flexible transfer sheet having the property of high lateral conductivity under conditions of high relative humidity comprising the steps of superposing a conductive sheet on the flexible transfer sheet overlying the powder image, superposing a pressure pad on the conductive sheet whereby the conductive sheet is pressed into intimate surface contact with the flexible transfer sheet, and establishing a potential difference between the transfer sheet and the plate by forming an electrical circuit between the conductive sheet and the conductive backing of the xerographic plate.

3. In the process of xerography wherein an electrostatically charged, flat xerographic plate comprising a photoconductive insulating layer on a conductive backing is exposed to a light image to form an electrostatic latent image on the plate surface corresponding to the light image and the plate is developed with oppositely charged particles of developing material to form a xerographic powder image on the plate surface corresponding to the light image and the powder image is transferred to a transfer sheet superposed thereon, the improvement for effecting said powder image transfer under conditions of high relative humidity fromthe xerographic plate to a flexible transfer sheet having the property of high lateral conductivity under conditions of high relative humidity comprising the steps of superposing a conductive sheet on the flexible transfer sheet overlying the powder image, superposing a pressure pad on the conductive sheet whereby the conductive sheet is pressed into intimate surface contact with the flexible transfer sheet, establishing a potential difference between the transfer sheet and the plate by forming an electrical circuit between the conductive sheet and the conductive backing of the xerographic plate, whereby a copious electrical charge is distributed via the conductive sheet to the entire surface area of the transfer sheet and in excess of the rate of dissipation of the charge by the lateral conductivity of the transfer sheet, and separating the transfer sheet from the xerographic plate immediately thereafter.

References Cited in the file of this patent UNITED STATES PATENTS 6 Prest June 3, 19 47 Hooper July 3, 1951 Copley May 5, 1953 Sabel et al. July 27, 1954 McNaney Feb. 28, 1956 Fitch Sept. 24, 1957 OTHER REFERENCES 8-25; page 18 particularly relied upon.

(Photostat copy in Division 67.) 

1. IN THE PROCESS OF XEROGRAPH WHEREIN AN ELECTROSTATICALLY CHARGED, FLAT XEROGRAPHIC PLATE COMPRISING A PHOTOCONDUCTIVE INSULTING LAYER ON A CONDUCTIVE BACKING IS EXPOSED TO A LIGHT IMAGE TO FORM AN ELECTROSTATIC LATENT IMAGE ON THE PALTE SURFACE CORRESPONDING TO THE LIGHT IMAGE AND THE PLATE IS DEVELOPED WITH OPPOSITELY CHARGED PARTICLES OF DEVELOPING MATERIAL TO FORM A XEROGRAPHIC POWDER IMAGE ON THE PLATE SURFACE CORRESPONDING TO THE LIGHT IMAGE AND THE POWDER IMAGE IS TRANSFERED TO A TRANSFER SHEET SUPERPOSED THEREON, THE IMPROVEMENT FOR EFFECTING SAID POWDER IMAGE TRANSFER UNDER CONDITIONS OF HIGH RELATIVE HUMIDITY FROM THE XEROGRAPHIC PLATE TO A FLEXIBLE TRANSFER SHEET HAVING THE PROPERTY OF HIGH LATERAL CONDUCTIVITY UNDER CONDITIONS OF HIGH RELATIVE HUMIDITY COMPRISING THE STEPS OF SUPERPOSEING A CONDUCTIVE SHEET ON THE FLEXIBLE TRANSFER SHEET OVERLYING THE POWDER IMAGE, PRESSING THE CONDUCTIVE SHEET INTO INTIMATE SURFACE CONTACT WITH THE FLEXIBLE TRANSFER SHEET, AND ESTABLISHING A POTENTIAL DIFFERENCE BETWEEN THE TRANSFER SHEET AND THE PLATE BY FORMING AN ELECTRICAL CIRCUIT BETWEEN THE CONDUCTIVE SHEET AND THE CONDUCTIVE BACKING OF THE XEROGRAPHIC PLATE. 